In today’s modern manufacturing sector, Safety is more than just a checkbox that needs to be complied with; Safety is an engineering discipline that needs to be foundational to the excellence of the operation and protection of the people working within the manufacturing environment. Safety in industrial automation needs to be smart & layered, with the right components being carefully selected, integrated, and added to the architecture of safety. Of the numerous components available, Touch Switches, Proximity Sensors, and Limit Switches are the most common critical components and are most often used.
For Safety Engineers, Plant Managers, and System Integrators, the ability to understand the configuration, application, and integration of these devices into machinery and production lines is critical to achieving very high International Safety Standards with no reduction in productivity
The Role of Functional Safety Standards
A major consideration that needs to be taken into account is the governing safety standards that need to be complied with in the first place prior to designing an architecture of safety. These Safety Standards are the foundation of the Safety System, which aims to achieve a measurable reduction of risk.
-
Key Standards: ISO 13849 and IEC 62061: These two standards represent the main breadth of international standards for the safety of machines and go from just component selection to looking at an entire Safety Function—a particular operation such as stopping a motor, performed to reduce risk, and initiated by a sensing device, such as a safety gate switch. These require designers to ascertain what the necessary Performance Level (PL) or the Safety Integrity Level (SIL) is for each individual function, which sets the criteria for dependability and fault exclusion for the component(s) utilized.
-
Creating a Safety Function: Each Safety Function, as the name suggests, should be a process and complete a process. A Safety Function should complete the process: Sensing → Logic → Action. Sensing devices (touch switches, safety sensors) see a hazardous state. Then a safety logic device (such as a safety relay or safety PLC) handles this signal. Then an actuator (a contactor, drive) makes the safe state (e.g., removes power). Each device in this chain must be used for safety duties, which often means having the appropriate certification.
-
Verifying the Performance of Safety System: Standards require documented risk assessment and proof that the safety system in place achieves the desired PLr. This entails estimating the Mean Time To Dangerous Failure (MTTFd), and for each component of the safety loop, the safety circuit designs, the diagnostic coverage (D), the common cause failure (CCF), and the total for the safety loop.
Touch Switches: Making Human to Machine Interaction More Safe and Clean
Touch switches (especially the capacitive and piezoelectric types) are modern and versatile human-to-machine interfaces that have proven reliability and can be fully sealed to protect from the environment.
-
Safety Critical Applications: Touch switches are not safety rated, and that is the case for most of the available device types. However, some switches are rated and can be classified as enabling or hold-to-run control switches. These can be three-position switches, and one of the typical applications is as a safety device for teaching controllers on a robot. The machine can only be run while the operator pushes the switch to the center position; releasing or fully pressing the switch will trigger the machine to go to a safe stop.
-
Integration into Safety Circuits: Safety touch switches use positive-guided contacts or redundant logic outputs monitored by a safety system. This means in the event of a closed-weld contact (a potentially hazardous failure), the monitoring logic will recognize the failure and trigger a safe state to ensure that the system remains operational.
Proximity Sensors for Non-Contact Safety Sensing
When engineered and implemented as safety components, proximity sensors serve non-contact safety perimeter monitoring, and access control, while maintaining the uninterrupted flow of operations.
-
Safety Light Curtains and Laser Scanners: These are specific photoelectric proximity sensors that use detection fields that are not visible. An access control light curtain in front of a robotic work cell will cause the robot to go into a stop condition if a beam is interrupted. Safety laser scanners demonstrate the ability to establish a greater variety of two-dimensional warning and protective zones on the floor around mobile equipment like AGVs or around machines, providing for speed reductions in warning zones and full stops in protective zones.
-
Safe Position Sensing with RFID: A safety-rated RFID system uses proximity technology so that a guard door is not only closed, but locked, before any machine operates. The safety controller captures a unique code from the tag on the guard, ensuring a signal that the right guard is fastened in position.
The Everlasting Role of Limit Switches in Safety Interlocks
The mechanical limit switch is a workhorse for safety interlocks. It is simple and is positively actuated. The interlocks also provide direct physical feedback.
-
Guard interlocking and door monitoring: Tongue-operated safety interlock switches are a typical example of a guard door and access panel monitoring solution. The interlock is a type of actuator. The guard is opened, and the actuator is released. This changes the contact state of the switch and sends a signal to the safety controller, and the controller goes to a stop state. The robust design allows no bypasses or tampering.
-
Positive actuation and fault detection: Safety limit switches contain force-guided (or positively guided) relay contacts. This mechanical linkage means that if the Normally Closed (NC) safety contact stays closed, the Normally Open (NO) auxiliary contact cannot close, yielding a fault condition that can be detected by the monitoring safety relay.
-
End-of-travel and over-travel safety: In addition to guarding, limit switches can serve as redundant end-of-travel sensors for linear actuators or lifts. This provides a hardwired backup to primary position sensors to avert mechanical overrun and damage or hazards. Critical fail-safe positioning requires robust and precisely engineered components. System designers often procure specialized variants such as a heavy-duty safety limit switch from OMCH for demanding applications.
Best Practices and System Integration for Creating a Safety Architecture
Integrating the safety components poorly results in the best components being ineffective. Using a system-level approach makes certain each constituent part is able to work together to minimize risk.
-
Selecting Optimal Safety Logic Device: A touch switch, safety sensor, and limit switch each serve a different purpose, and are wired for a particular safety controller. There are safety relays for simple, single functions, while complex, programmable safety systems for multiple zones and interlinked machines use safety PLCs. The logic device incorporates dual-channel cross-monitoring, self-testing, and dependable output switching.
-
Wiring and Installation Integrity: The safety circuits require a higher level of care. For monitored inputs, dual channel wiring must be employed. Safety and control wiring must be separated in cable ducts to avoid cross-talk and damage. The components must be firmly fastened to avoid displacement from vibrations, which would shift sensing fields or actuator alignment.
-
Documentation, Validation, and Regular Testing: Every safety system should have sufficient documentation, which includes things such as safety function validation, a list of parts, and circuit diagrams. It is also critical that the system is tested functionally at the time of commissioning, as well as at regular intervals afterwards, as prescribed by the standards, in order to demonstrate ongoing reliability. This includes testing to see if guard interlocks initiate the appropriate stop response and monitoring failures to see if they are detected.
For B2B companies, the construction of a safety system of well-designed and appropriately certified components is a legal, ethical, and financially sound necessity. Using touch switches to allow operators to make command inputs, proximity sensors to ensure non-contact-zone protection, limit switches to provide positive-guard interlocking, and all of these devices to a certified safety controller, engineers are able to establish a complete safety system.
This multi-layer safety approach protects people and also expensive capital equipment from damage, and helps reduce the costly downtime associated with safety incidents. Working with a supplier such as OMCH for the essential components of safety systems helps ensure that the safety systems meet the required international certifications, as well as the reliability and safety traceability that is essential for the proper functioning of the systems.